1. Field of the Invention
This invention relates to a filtering device and a differential signal transmission circuit, more particularly to a filtering device and a differential signal transmission circuit capable of suppressing common-mode noises upon transmission of a differential signal.
2. Description of the Related Art
Differential signal transmission has been widely used in high-speed digital systems. However, a differential signal may accompany unwanted common-mode noises. For a high-speed data link, a cable is necessary to transmit the differential signal between two different electronic apparatuses. When the common-mode noises are coupled to the cable, the cable is excited to behave as an electromagnetic interference (EMI) antenna. Therefore, suppressing the common-mode noises upon transmission of the differential signal is necessary to solve the EMI problem associated with the cable.
Some conventional filtering devices capable of suppressing common-mode noises upon transmission of a differential signal employ patterned grounding structures, such as those disclosed in “An Embedded Common Mode Suppression Filter for GHz Differential Signals Using Periodic Defected Ground Plane,” IEEE Microwave and Wireless Components Letters, vol. 18, no. 4, pp. 248-250, April 2008 and “A Novel Wideband Common-Mode Suppression Filter for GHz Differential Signals Using Coupled Patterned Ground Structure,” IEEE Transactions on Microwave Theory and Technology, vol. 57, no. 4, pp. 848-855, April 2009. Although each of the aforesaid filtering devices has a relatively low cost, and is advantageous in terms of common-mode noises suppression over a wideband frequency range, it is disadvantageous in the following ways: a) it can not be miniaturized because one of the length and the width of the patterned grounding structure must be one half or one quarter of the wavelength of the differential signal, and b) its performance will be degraded with the inclusion of a shielding structure beneath the patterned ground structure.